I. Field of the Invention
This invention is directed to wireless communication systems. More specifically, this invention is directed to methods for analyzing and improving the locations for hard handoffs in wireless communication systems.
II. Description of Related Art
In a typical cellular radio communications system (wireless telecommunications network), an area is divided geographically into a number of transmission areas, or cell sites. Each cell site is defined by a radio frequency (RF) radiation pattern from a base transceiver station (BTS) antenna. The base station antennae in the cells are in turn coupled to a base station controller (BSC), which is then coupled to a telecommunications switch or gateway, such as a mobile switching center (MSC). The MSC may then be coupled to a telecommunications network such as the PSTN (Public Switched Telephone Network) or the Internet. Further, commonly owned BSCs for multiple base stations (such as those of a specific service provider) may be coupled to a base station manager (BSM) that handles communication between those BSCs for managing communication traffic in the wireless network.
When a mobile station (MS) such as a cellular telephone, pager, PDA, or appropriately equipped portable computer, is positioned in a cell, the MS communicates via an RF air interface with the BTS antenna of the cell. Thus, a communication path is established between the MS and the telecommunications network, via the RF interface, the BTS, the BSC and the MSC.
With the rapid growth in demand for wireless communications, the level of call traffic in most cell sites has increased dramatically in recent years. To help manage the call traffic, most cells in a wireless network are usually further divided geographically into a number of sectors, each defined by radiation patterns from directional antenna components of the BTS, or by BTS antennae. These sectors (which can be visualized ideally as pie pieces) can be referred to as “physical sectors,” since they are physical transmission/reception areas (hereafter “transmission area(s)”) of a cell site. Therefore, at any given time, an MS in a wireless network will typically be positioned in a given physical sector and will be able to communicate with the telecommunications network via the BTS serving that physical sector.
In CDMA (Code Division Multiple Access) systems, which are known, each physical sector is distinguished from geographically adjacent physical sectors by a PN offset number or key. PN offsets are pseudo-noise (e.g., deterministic “noise-like” information) that is inserted in the carrier signal for the corresponding sector. When a mobile station is in a particular physical sector, communication between the mobile station and the BTS of the cell site are encoded by the physical sector's PN offset key, regardless of the carrier frequency being used. This allows the same carrier frequency to be used by geographically adjacent sectors with minimal interference between the sectors occurring.
In areas where wireless communication traffic is particularly high, cell sites in those areas may employ more than one carrier frequency for communicating with the mobile stations that are within its transmission/reception area boundaries. The number of carrier frequencies employed by a given cell site may depend on various factors, such as the volume of communication traffic expected. For example, in a congested urban location, cell sites might be designed to employ two or more carrier frequencies, while in more sparsely populated rural areas, cell sites might employ only one carrier frequency.
Cell sites that employ more than one carrier frequency may be termed as having a “primary” carrier frequency and one or more “overlay carrier frequencies.” Typically, the primary frequency is the carrier frequency that is implemented by all the cell sites in a particular geography, such as in a particular city and its surrounding area. Overlay frequencies are then implemented by the cell sites in the geography that carry more traffic than may be handled using only the primary frequency. It will be appreciated that implementing overlay carrier frequencies is relatively expensive. This expense is due to additional equipment costs, maintenance, licensing fees (such as charged by the Federal Communication Commission), among other costs.
In normal operation, when a mobile station is operating on a given frequency and moves into a new physical sector operating on the same frequency, the call will typically continue on that same frequency in the new physical sector (e.g., if the new physical sector is controlled by the same BSC or both physical sectors are coupled with the same BSM through respective BSCs). Through communication with the BSC (e.g., and, indirectly, the base station manger) the mobile station will simply switch to the PN offset key of the new physical sector in order to complete the handoff from one physical sector to the next. This process is termed as a “soft handoff”, as the call is maintained on the same carrier frequency and only the pseudo-noise used is changed.
When a mobile station is operating on an overlay frequency carrier signal and the mobile station moves into (or toward) a new physical sector that does not implement that particular overlay frequency as a carrier signal (e.g., a cell site which handles less traffic), a process known as a “hard handoff” must occur to maintain the call. A hard handoff comprises a “call” (e.g., the communication path between the MS and the BTS) being moved from a carrier signal of the overlay frequency to a carrier signal of the primary frequency or another overlay frequency.
Due to the cost associated with implementing an overlay frequency, it is desirable that calls using the overlay frequencies be maintained on the overlay frequency as long as possible, so as to increase the utilization of the overlay frequency. Therefore, it is desirable that hard handoffs occur close to the transmission/reception boundary of the cell site implementing the overlay frequency. However, executing a hard handoff too close to the transmission area boundary may result in the call being dropped.
Currently, hard handoff locations are adjusted by performing iterative drive tests of cellular coverage areas. During a drive test, pilot signal quality information is collected using, for example, a pilot signal scanner. For CDMA systems, pilot signals are transmitted by the cell sites to facilitate call initiation as well as to facilitate handoffs of calls from one sector/cell site to another sector/cell site, among other functions. Each pilot signal has a corresponding PN offset that is used to identify it. For a cell site that does not implement a particular overlay frequency that is implemented by a neighboring cell site/sector or cell sites/sectors, a pilot signal for the overlay frequency is still employed to facilitate hard handoffs between sectors. As is known, hard handoff locations are determined based on the relative quality of the pilot signals in the coverage area.
If it is determined that hard-handoffs are occurring at undesirable locations (e.g., too far within the cell site implementing the overlay frequency or too close to the transmission/reception area boundary), adjustments are made to the wireless network, such as adjustments to pilot signal strengths. After making such adjustments, the drive test is redone to re-collect pilot signal quality information and the process is repeated. This process of iterative drive testing is time-consuming and expensive. In this regard, it may take five to seven drive tests, or more, in a given coverage area to satisfactorily adjust the hard handoff locations. Therefore, based on the foregoing, techniques for adjusting hard handoff locations and the cost associated with iterative drive tests are desirable.